Membrane materials used for chemical separation should be as thin as possible to increase the diffusion rate, mechanically strong to survive pressure and erosion conditions, chemically inert to the electrolytes used, have a very low hydraulic permeability to minimize unwanted diffusion, insensitive to temperature extremes likely to be encountered, immune from swelling or other dimensional changes, and of low cost. Further, membranes should be immune to organic fouling, and should, in many applications, be highly ion-selective. In addition, where used in any electro-dialysis process, the membrane should have a low electrical resistance permitting a high current density, and should have a high dielectric constant to eliminate problems of electric breakdown.
Efforts in the past have attempted to find materials which will satisfy the above requirements to the greatest extent possible, with optimization directed toward various specific applications.
For example, reverse osmosis membranes must withstand pressures up to 1000 psi. Such membranes are usually permeable only to water, and accordingly are not used for separating other components of a solution, as is often required.
Membranes used in the production of sodium hydroxide are hydraulically permeable, allowing considerable salt to pass through and thus contaminate the desired product. Membranes which will prevent such contamination and withstand the chemical conditions in the cell have not been available in the past.
Membranes used for ion selection, often called ion-exchange or permselective membranes, are expensive and are subject to organic fouling. Further, the selectivity, while necessary in a particular application, precludes the use of such membranes in applications where non-selectivity is desired.
Membranes used for osmotic (dialysis) processes are limited to those materials which exhibit high permeability to the constituents desired to be transferred through the membranes. Accordingly, the materials available for such membranes are limited in variety and hence are limited in mechanical and chemical properties.
In any membrane exposed to an electrolyte containing organic matter or other high molecular weight material, such material may be selectively absorbed by the membrane, causing clogging which reduces the desired permeability, and may cause the surface to become hydrophobic, thereby preventing the transport of water across the membrane.
Most of the above-mentioned problems associated with dialysis, reverse osmosis, and electrodialysis are a result of the limited range of materials available for membranes which have permeabilities high enough for practical purposes.